One: Both platforms by providing a platform for public servants... cough-laugh politicians, there are good arguments to be made that in some cases they are public forums. I heard this argument put very well by a law podcast when discussing the "Trump can't block people on twitter" saga.
Two: Given the governments heavy involvement with these companies behind the scenes, I think there is also a compelling argument that that heavy involvement could change the view of the companies as purely private. (From initial funding via CIA VC In-Q-Tel to National Security Letters and other various entangling structures usually under the banner of "national security")
Great article, just wanted to add those two things.